243. PPUCTRL controla el incremento de PPUDATA

PPUCTRL controla en cuánto incrementa PPUDATA vram_addr.

Lección 243 de 356 · tests/chapter_03_ppu_memory_and_graphics_data/test_243_ppuctrl_controls_ppudata_increment.py

Referencia

https://www.nesdev.org/wiki/PPU_registers#PPUCTRL

Archivo a actualizar

emulator/ppu/ppu.py

Constante a añadir

CTRL_VRAM_INCREMENT_BY_32 = 1 << 2

Por qué existe este paso

PPUDATA ($2007) accede a la memoria de la PPU en la dirección de VRAM interna actual. Después de cada acceso, la PPU incrementa esa dirección.

La cantidad de incremento se controla con el bit 2 de PPUCTRL ($2000):

bit 2 clear -> increment by 1
bit 2 set   -> increment by 32

Por qué es importante

El incremento de 1 es útil para escribir a lo largo de una fila consecutiva de memoria de la PPU. El incremento de 32 es útil para escribir hacia abajo en una columna de la memoria de nametable.

Pseudocódigo de implementación sugerido

CTRL_VRAM_INCREMENT_BY_32 = 1 << 2

case 0x2007:
    self.data = value
    self.ppu_bus.write(self.vram_addr, value)

    increment = 32 if self.ctrl & CTRL_VRAM_INCREMENT_BY_32 else 1
    self.vram_addr = (self.vram_addr + increment) & 0x3FFF

Importante

Sigue enmascarando vram_addr con 0x3FFF porque el espacio de direcciones de la PPU es de 14 bits:

$0000-$3FFF

Fuera del alcance

otros bits de PPUCTRL
habilitación de NMI
selección de pattern table
selección de nametable base
renderizado

Ejecutar esta lección

uv run pytest tests/chapter_03_ppu_memory_and_graphics_data/test_243_ppuctrl_controls_ppudata_increment.py -v

Implementación de ejemplo

Pseudocódigo de implementación sugerido

CTRL_VRAM_INCREMENT_BY_32 = 1 << 2

case 0x2007:
    self.data = value
    self.ppu_bus.write(self.vram_addr, value)

    increment = 32 if self.ctrl & CTRL_VRAM_INCREMENT_BY_32 else 1
    self.vram_addr = (self.vram_addr + increment) & 0x3FFF

Ver código del testtest_243_ppuctrl_controls_ppudata_increment.py

from emulator.ppu.ppu import CTRL_VRAM_INCREMENT_BY_32, PPU


def set_ppuaddr(ppu: PPU, addr: int) -> None:
    """Set PPU.vram_addr through the public $2006 two-write interface."""
    ppu.write_register(0x2006, (addr >> 8) & 0xFF)
    ppu.write_register(0x2006, addr & 0xFF)


def test_ppuctrl_vram_increment_constant_exists():
    """
    Objective:
    Name PPUCTRL bit 2, which selects PPUDATA increment-by-32 mode.
    """
    assert CTRL_VRAM_INCREMENT_BY_32 == 1 << 2


def test_ppudata_increments_vram_addr_by_one_when_increment_bit_is_clear():
    """
    Objective:
    With PPUCTRL bit 2 clear, PPUDATA should increment vram_addr by 1.
    """
    ppu = PPU()
    ppu.write_register(0x2000, 0x00)
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xAA)

    assert ppu.vram_addr == 0x2001


def test_ppudata_increments_vram_addr_by_32_when_increment_bit_is_set():
    """
    Objective:
    With PPUCTRL bit 2 set, PPUDATA should increment vram_addr by 32.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_VRAM_INCREMENT_BY_32)
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xBB)

    assert ppu.vram_addr == 0x2020


def test_ppudata_increment_by_32_still_writes_to_original_address_before_incrementing():
    """
    Objective:
    PPUDATA should write to the old vram_addr first, then increment.

    Why:
    The increment affects the next PPUDATA access, not the current one.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_VRAM_INCREMENT_BY_32)
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xCC)

    assert ppu.ppu_bus.read(0x2000) == 0xCC
    assert ppu.vram_addr == 0x2020


def test_ppudata_increment_wraps_to_14_bit_ppu_address_space():
    """
    Objective:
    vram_addr should stay inside the PPU $0000-$3FFF address range after
    incrementing.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_VRAM_INCREMENT_BY_32)
    set_ppuaddr(ppu, 0x3FF0)

    ppu.write_register(0x2007, 0xDD)

    assert ppu.vram_addr == ((0x3FF0 + 32) & 0x3FFF)

Funciones de test (5)

test_ppuctrl_vram_increment_constant_exists
test_ppudata_increments_vram_addr_by_one_when_increment_bit_is_clear
test_ppudata_increments_vram_addr_by_32_when_increment_bit_is_set
test_ppudata_increment_by_32_still_writes_to_original_address_before_incrementing
test_ppudata_increment_wraps_to_14_bit_ppu_address_space